How to use dbms_session to set client_identifier in your session

Logged in as user scott, I set the following in my session:

EXEC DBMS_SESSION.SET_IDENTIFIER (client_id=>'vegardk');

Verify that your session info is set:

SELECT SYS_CONTEXT( 'USERENV', 'CLIENT_IDENTIFIER' ) "client identifier", SYS_CONTEXT( 'USERENV', 'CURRENT_USER' ) "current user" 
FROM DUAL;

client identifier	current user
vegardk	SCOTT

The same information can be found in v$session, columnn CLIENT_IDENTIFIER.

There is also another column in v$session called CLIENT_INFO.
A value for this column can be generated by calling a different procedure, dbms_application_info.set_client_info.

For example:

exec dbms_application_info.set_client_info('Execute summary');

Check v$session:

SELECT USERNAME,CLIENT_IDENTIFIER,CLIENT_INFO
FROM V$SESSION 
WHERE USERNAME='SCOTT';

Result:

USERNAME	CLIENT_IDENTIFIER	CLIENT_INFO
SCOTT	vegardk	Execute summary

A good way to pick up up any relevant session information already set is to use NVL:

SELECT NVL(SYS_CONTEXT( 'USERENV', 'CLIENT_IDENTIFIER' ), SYS_CONTEXT( 'USERENV', 'CURRENT_USER' )) 
FROM DUAL;

If client_identifier is set, we pick up that value. If not, we use the current_user, which is always set.
This could also be used as a default value for a column definition:

  CREATE TABLE T1 (
  ...
  LAST_CHANGED_BY     VARCHAR2(30 CHAR)         DEFAULT NVL(SYS_CONTEXT( 'USERENV', 'CLIENT_IDENTIFIER' ), SYS_CONTEXT( 'USERENV', 'CURRENT_USER' )) NULL
 );